1. Technical Field
The present invention relates to a printing method and to a printer for repeatedly printing desired variable images on recording substrates by utilizing an electrophoretic display method.
2. Background Art
Electrophoretic displays (hereinafter referred to as xe2x80x9cEPDxe2x80x9d) are a non-emitting type display which utilizes electrophoresis of charged particles (electrophoretic particles) dispersed in a dispersion medium, and they have characteristics which are not provided by liquid crystal displays, such as high contrast, wide viewing angle, long duration of display without electric power supply, reduced electric power consumption, and the like.
EPDs are known in Japanese Patent Publication, No. 52-28354 and Japanese Patent No. 2551783. According to a basic feature of the EPD, a medium dispersing electrophoretic particles therein is enclosed between a pair of opposing electrode layers of which at least one is transparent, and the surface of the transparent electrode layer serves as a viewing surface.
When a voltage for producing a display is applied between the electrode layers, a displaying movement, in which the electrophoretic particles are drawn to one of the electrode layers, is performed. A viewer recognizes the electrophoretic particles or the medium as an image. In an actual image display, one of the electrode layers is divided into a specific display pattern, and the polarity of voltage applied to each divided electrode layer is separately controlled. In this way, contrast between the electrophoretic particles and the medium is produced, thereby forming a desired image.
The EPD is considered to have rewritable properties in which erasing and printing of images can be repeated usually by controlling the polarity of the voltage applied between a pair of electrode layers. The inventors have intensively researched the possibility of applying a new rewriting method utilizing the electrophoretic display system. Printing substrates for rewriting may be applied to recording media, and the like, to which variable information is recorded. For example, point cards to which points corresponding to purchase amounts are added and are displayed, consultation cards on which the next consultation date and time are displayed, and other types of cards may be mentioned. The electrophoretic display system may be applied to display contents of data or reminders of the capacity of floppy disks (FD), optical magnetic recording disks (MO), and Zip disks, and the like, which are used for backup or delivery of various types of data in computer environments.
In order to apply the electrophoretic display system to such recording media, it may easily be anticipated that the EPD may be installed on the surface of the media. In such a manner, however, one of the electrode layers, which is divided into a display pattern, is relatively complicated, and the circuit for applying a voltage to the divided electrode layer is complicated. This makes the cost of the media very high, and use thereof may therefore not be practical.
The invention has been made in consideration of the above situation. An object of the present invention is to provide an electrophoretic printing method and an apparatus which can easily rewrite images on printing substrates such as cards and magnetic recording media at low cost, and the cost of the printing substrates can be held down.
The inventors have noted that, without an electrode layer for causing displaying movement, the remaining other electrode layer and the dispersing system can be only a paper-like recording medium, which is low cost. The inventors found that images can be displayed similarly to those in EPDs and that the objects of the invention can be effectively accomplished by separating the electrode layer for causing displaying movement and by contacting that electrode layer to the dispersion system to apply a voltage across the two electrodes during printing.
The invention has been made based on the above. The invention provides an electrophoretic printing method comprising: layering a back electrode and a dispersion system containing electrophoretic particles on a printing substrate to form a printing layer; and providing a printing head separately from the printing substrate. The printing head has a front electrode which is a counterpart of the back electrode and which has a predetermined printing pattern. The front electrode is brought into contact with the printing layer formed on the printing substrate to oppose the back electrode. Voltage is applied between the front electrode and the back electrode so as to change a distribution of the electrophoretic particles according to the polarity of the voltage and to form a desired image on the dispersion system.
According to the invention, the printing head contacts the printing layer formed on the printing substrate and applies a voltage between the front electrode and the back electrode so that the dispersing system performs displaying movement and repeatedly forms images. That is, the invention can perform rewriting. The printing substrate referred to is an object to be printed, for which the above-mentioned cards and magnetic recording media may be mentioned as examples.
In the invention, the printing layer comprises the back electrode and the dispersion system serves as a medium for printing. The printing layer may be formed on plural printing substrates, which may be printed by one printing head. Therefore, images can be easily rewritten on the printing substrates at low cost, and the cost of the printing substrate can be held down. Furthermore, clear images can be obtained since the dispersing system can be viewed directly rather than the dispersing system being viewed through a transparent electrode layer as in EPDs, so that the quality of the images can be increased. Moreover, since a printing method by application of voltage is used rather than a printing method using magnetism or heating, printing can be safely performed even if the printing substrate is a magnetic recording medium which is affected by magnetism or a medium such as a rewritable CD which is readily affected by heat.
The front electrode in the invention is divided into a specific printing pattern. A desired image is formed on the dispersion system by selectively applying voltage to the front electrode. On the other hand, the image is erased by applying voltage to the front electrode entirely.
When the front electrode consist comprises only the printing electrode having a specific printing pattern, the printing pattern of the front electrode must be opposed to the printed image with high precision during contacting the printing head to the printing layer for rewriting in order to completely erase the printed image. However, errors readily occur in such positioning, and further printing may be performed on an incompletely erased portion, and the images may therefore be unclear.
Therefore, in order to avoid the above-mentioned problems, the front electrode may comprise a printing electrode for forming a variable desired image and a marginal electrode disposed around the printing electrode to form a marginal image. According to the preferable feature, incomplete erasure can be avoided since the marginal electrode erases the circumference of the printed portion printed by the printing electrode. As a result, clear rewritten images can be obtained even if a printed image is inadequately positioned relative to the printing head.
According to a preferable feature of the invention, the printing head may comprise a contact electrode for applying voltage to the back electrode at substantially the same plane as the printing electrode. Since the printing surface of the printing head comprises the printing electrode and the contact electrode, the design of the electrodes, specifically the contact electrode, can be simplified. Moreover, the feature allows simultaneous thrusting by the electrodes to the back electrode and the printing surface, and reliable printing can be performed.
According to a preferable feature of the invention, at least one pre-printing cycle may be performed on the dispersion system before a desired image is formed thereon. In the pre-printing, the entire surface of the dispersion system is printed, and then the entire surface of the dispersion system is erased. The pre-printing cycle activates the electrophoresis of the electrophoretic particles in the dispersion system, so that the subsequent printing movement of the electrophoretic particles, that is, the drawing of the electrophoretic particles to the front electrode or the back electrode is reliably performed. As a result, the printed images have a high contrast appearance.
According to a preferable feature of the invention, the voltage applied between the electrodes is divided into pulses, and the pulses are repeatedly applied thereto. When the voltage pulses are repeatedly applied, the electrophoretic particles are repeatedly electrophoresed in the vicinity of the printing electrode to which the electrophoretic particles are drawn, and are activated similarly to the case in which the pre-printing is performed.
The invention further provides an electrophoretic printing apparatus for printing on a printing substrate having a printing layer thereon comprising a back electrode and a dispersion system containing electrophoretic particles. The apparatus comprises: a printing head provided separately from the printing substrate. The printing head has a front electrode which is a counterpart of the back electrode and which has a predetermined printing pattern. The front electrode is brought into contact with the printing layer formed on the printing substrate to oppose the back electrode. Voltage is applied between the front electrode and the back electrode so as to change a distribution of the electrophoretic particles according to the polarity of the voltage and to form a desired image on the dispersion system.
The electrophoretic printing apparatus has the same advantages and may have the same preferable features as those provided by the above-mentioned printing method.